Axial coordination of pyridyl-containing pentacenes to porphyrins

Article abstract of DOI:10.1080/00958972.2015.1057711

A pair of pentacenes that are functionalized in the 6-position with either a 3- or 4-pyridyl group via a triazole linker have been used to form complexes with a tetra(aryl)ruthenium(II) porphyrin through axial coordination. The pentacene-porphyrin dyads 5

Full text of DOI:10.1080/00958972.2015.1057711

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Axial coordination of pyridyl-containing
pentacenes to porphyrins
Andreas R. Waterloo, Rainer Lippert, Norbert Jux & Rik R. Tykwinski
To cite this article: Andreas R. Waterloo, Rainer Lippert, Norbert Jux & Rik R. Tykwinski (2015)
Axial coordination of pyridyl-containing pentacenes to porphyrins, Journal of Coordination
Chemistry, 68:17-18, 3088-3098, DOI: 10.1080/00958972.2015.1057711
To link to this article: http://dx.doi.org/10.1080/00958972.2015.1057711
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Date: 01 October 2015, At: 04:04

Journal of Coordination Chemistry, 2015
Vol. 68, Nos. 17–18, 3088–3098, http://dx.doi.org/10.1080/00958972.2015.1057711
Axial coordination of pyridyl-containing pentacenes to
porphyrins
ANDREAS R. WATERLOO, RAINER LIPPERT, NORBERT JUX* and
RIK R. TYKWINSKI*
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM),
University of Erlangen-Nürnberg (FAU), Erlangen, Germany
(Received 4 April 2015; accepted 23 April 2015)
A pair of pentacenes that are functionalized in the 6-position with either a 3- or 4-pyridyl group via
a triazole linker have been used to form complexes with a tetra(aryl)ruthenium(II) porphyrin through
axial coordination. The pentacene–porphyrin dyads 5 and 6 have been structurally characterized
through a combination of spectroscopic techniques. UV–vis spectroscopy shows that the absorption
profiles of the two chromophores, the porphyrin and the pentacene, are complementary, providing
absorptions throughout the UV and visible regions. While the dyads are reasonably stable in the
solid state under ambient conditions they are, unfortunately, only stable in solution for hours when
exposed to light and air.
Keywords: Functionalized pentacene; Axial coordination; Porphyrin; CuAAC reaction
1. Introduction
Metal–pyridine complexation has been extensively used in coordination chemistry [1], espe-
cially in the formation of porphyrin-containing architectures [2–4]. Axial complexation of
pyridyl moieties to metalloporphyrin centers can lead to supramolecular assemblies such as
cages [5], oligomers [6], polymers [7], wires [8], or tweezers [9]. Complementing the
intriguing properties of porphyrins are those of polycyclic aromatic hydrocarbons such as
pentacenes. While unsubstituted pentacene is relatively unstable under ambient conditions
and only sparingly soluble in common organic solvents [10], substituted pentacenes have
recently emerged as a versatile platform to provide a range of useful molecular materials.
*Corresponding authors. Email: norbert.jux@fau.de (N. Jux); rik.tykwinski@fau.de (R.R. Tykwinski)
Dedicated to Professor Rudi van Eldik on the occasion of his 70th birthday.
© 2015 Taylor & Francis

Pentacene–porphyrin dyads
3089
N
N
N
1.
N
N
H
N3
N
N
N
N
"CuAAC"
MeO
or
2. SnCl2•2H2O
"reductive
elimination"
OMe
Sii-Pr3
1
Sii-Pr3
2
Sii-Pr3
Figure 1. Pyridyl-containing pentacenes 1 and 2 formed by CuAAC [16].
As a result, acenes have been widely studied as high-performance organic semiconductors
[11–14].
We have recently shown that the Cu(I)-catalyzed alkyne–azide cycloaddition (CuAAC)
[15] offers a possibility to easily alter the substitution pattern of 6,13-disubstituted pentace-
nes under mild reaction conditions [16]. In addition to allowing the incorporation of aryl
and alkyl groups, the CuAAC allowed for the introduction of pyridyl moieties onto the pen-
tacene framework, as in 1 and 2 (figure 1) [16]. It was hypothesized that a combination of
pyridyl-containing pentacene derivatives 1 and 2 and a metalloporphyrin via axial coordina-
tion could lead to pentacene–porphyrin dyads with potentially interesting electronic and/or
optical properties, based on the complementary chromophores of the two components of
the dyad [17]. The results of this study are reported herein.
2. Experimental
2.1. Materials and methods
Compounds 1 [16], 2 [16], and 3 [18] were synthesized as reported. Chemicals were pur-
chased in reagent grade from commercial suppliers and used without purification. Unless
otherwise stated, all reactions were performed in standard, dry glassware under nitrogen.
All solvents were dried and/or distilled prior to use. Dry, deoxygenated methylene chloride
was distilled from calcium hydride; dry, deoxygenated hexanes were distilled from sodium.
NMR spectra were recorded on a Bruker Avance 300 operating at 300 MHz (1H NMR)
and 75 MHz (13C NMR) or on a Bruker Avance 400 operating at 400 MHz (1H NMR) and
100 MHz (13C NMR) at rt. The signals were referenced to residual solvent peaks (δ in parts
per million; ppm). Coupling constants are reported as observed ( 0.5 Hz). For simplicity,
the coupling constants for the aryl protons of para-substituted aryl groups have been
reported as pseudo first order (i.e. doublets), even though they are second-order (AA′XX′)
spin systems. Mass spectra were obtained from a Bruker Daltonik maXis 5G UHR ESI-
TOF instrument. IR spectra were recorded on a Varian-660 IR spectrometer in ATR mode.
UV–vis absorption spectra were acquired at rt using a Varian Cary 5000 spectrophotometer;
λmax in nm (ε in L mol−1 cm−1).
2.2. Synthesis
2.2.1. Pyridine-porphyrin dyad 4. RutBuPP·H2O 3 (50 mg, 0.050 mmol) was dissolved
in dry methylene chloride (10 mL). Dry pyridine (5 mL) was added and the reaction

3090
A.R. Waterloo et al.
mixture was stirred under ambient conditions for 3 h at rt. The reaction solution was evapo-
rated to dryness, and the residue was precipitated from chloroform/pentane to provide 4 as
a red solid (50 mg, 95%). IR (ATR): 3026 (w), 2957 (s), 2865 (s), 1946 (s), 1006 (s), 792
(s), 713 (s) cm−1; UV–vis (CH2Cl2) λmax (ε): 245 (25 500), 415 (111 000), 534 (11 500),
1
569 (6 000) nm; H NMR (300 MHz, C6D6): δ 9.02 (s, 8H), 8.21 (d, J = 9.8 Hz, 4H), 8.18
(d, J = 9.9 Hz, 4H), 7.59 (d, J = 7.9 Hz, 4H), 7.54 (d, J = 7.9 Hz, 4H), 4.96 (t, J = 7.5 Hz,
1H), 4.36 (t, J = 7.3 Hz, 2H), 1.88 (d, J = 5.1 Hz, 2H), 1.42 (s, 36H); 13C NMR (75 MHz,
C6D6): δ 181.4, 150.5, 145.0, 144.6, 140.8, 135.4, 134.7, 133.0, 124.4, 123.8, 122.8, 121.6,
35.0, 31.9; ESI HRMS (CH2Cl2/MeCN) Calcd for C66H65N5O102Ru ([M]+) m/z 1045.4227,
found 1045.4256.
2.2.2. Synthesis of pentacene–porphyrin dyads 1 and 2.
2.2.2.1. General procedure. A Schlenk flask was charged with either pyridyl-substituted
pentacene derivative 1 or 2 (1 equiv) and RutBuPP·H2O 3 (1 equiv). Dry, deoxygenated
benzene (5 mL) was added and the solution was stirred for 1 h at rt. The mixture was
heated to 50 °C and stirred for 2 h at that temperature. The reaction mixture was cooled to
rt and the solvent was removed under reduced pressure. The solid purple residue was redis-
solved in minimal amount of dry, deoxygenated methylene chloride and precipitated by
addition of a large amount of dry, deoxygenated hexanes. The solids were filtered under N2
and dried in vacuo to provide the products as purple solids.
2.2.2.2. Dyad 5. Pentacene 1 (12 mg, 0.020 mmol) and RutBuPP·H2O 3 (20 mg,
0.020 mmol) were used according to the General Procedure. Complex 5 was obtained as a
purple solid (21 mg, 67%). M.p. > 400 °C; UV–vis (CH2Cl2) λmax (ε): 309 (210 000), 346
(13 300), 415 (200 000), 496 (5 350), 534 (18 700), 571 (11 400), 622 (11 600) nm; IR
1
(ATR): 3029 (w), 2953 (s), 2862 (s), 2123 (m), 1949 (s), 1262 (s), 1006 (s) cm−1; H NMR
(300 MHz, C6D6): δ 9.70 (s, 2H), 9.05 (s, 8H), 8.43 (s, 2H), 8.38 (d, J = 2.0 Hz, 2H), 8.35
(d, J = 2.0 Hz, 2H), 8.11 (d, J = 2.0 Hz, 2H), 8.09 (d, J = 1.9 Hz, 2H), 7.99 (d, J = 8.7 Hz,
2H), 7.53–7.40 (m, 11H), 7.31 (d, J = 2.1 Hz, 2H), 7.29 (d, J = 2.1 Hz, 2H), 6.80 (s, 1H),
5.62 (d, J = 8.2 Hz, 1H), 4.30 (dd, J = 8.3, 5.7 Hz, 1H), 2.42 (d, J = 2.4 Hz, 1H), 1.95 (dd,
J = 5.7, 1.2 Hz, 1H), 1.46–1.45 (m, 21H), 1.34 (s, 36H). ESI HRMS (CH2Cl2/MeOH)
Calcd for C101H98N8O102RuSi ([M]+) m/z 1568.6676, found 1568.6715; Calcd for
C61H60N4RuO ([M–1]+) m/z 966.3822, found 966.3830.
2.2.2.3. Dyad 6. Pentacene 2 (10 mg, 0.017 mmol) and RutBuPP·H2O 3 (17 mg,
0.017 mmol) were used according to the General Procedure. Complex 6 was obtained as a
purple solid (20 mg, 74%). UV–vis (CH2Cl2) λmax (ε): 309 (133 000), 348 (8 200), 415
(125 000), 496 (3 420), 534 (11 600), 571 (7 100), 622 (7 300) nm; IR (ATR): 2955 (s),
1
2863 (s), 1966 (s), 1609 (s), 1350 (s), 1006 (s), 715 (s) cm−1; H NMR (300 MHz, C6D6):
δ 9.62 (s, 2H), 9.10 (s, 8H), 8.43 (d, J = 2.0 Hz, 2H), 8.40 (d, J = 2.0 Hz, 2H), 8.27 (s,
2H), 8.22 (d, J = 2.0 Hz, 2H), 8.20 (d, J = 2.0 Hz, 2H), 7.90 (d, J = 8.4 Hz, 2H), 7.68 (d,
J = 2.0 Hz, 2H), 7.65 (d, J = 2.0 Hz, 2H), 7.59 (d, J = 2.0 Hz, 2H), 7.56 (d, J = 2.1 Hz,
2H), 7.51 (d, J = 8.4 Hz, 2H), 7.03–6.88 (m, 5H), 6.30 (s, 1H), 4.72 (d, J = 7.1 Hz, 2H),
1.86 (d, J = 7.1 Hz, 2H), 1.42 (bs, 36H), 1.37–1.36 (m, 21H); ESI HRMS (CH2Cl2/MeOH)
Calcd for C101H98N8O2102RuSi ([M–CO + MeOH]+) m/z 1572.6620, found 1572.6989.

Pentacene–porphyrin dyads
3091
3. Results and discussion
3.1. Synthesis of pyridine-porphyrin complex
The synthesis envisioned for the desired porphyrin complexes would rely on a ligand
exchange reaction with Ru(II)-porphyrin 3 [18]. The use of tert-butyl substitution in the
para-position of the porphyrin should help to ensure solubility of the products, and it has
been shown previously that the nature of the aryl substituent in the meso-position has little
impact on the optical characteristic of the Ru-porphyrin, due to the orthogonal conformation
of the aryl rings [18]. For comparison and as reference compound, the pyridine complex of
Ru(II)-porphyrin 3 was generated first. To this end, Ru(II)-porphyrin 3 was dissolved in
methylene chloride and treated with excess of pyridine (scheme 1). After work up, the
desired pyridine complex 4 was isolated in excellent yield (95%). The solubility of pyridine
complex 4 is remarkably increased compared to ruthenium porphyrin 3.
3.2. Characterization of pyridine-porphyrin complex
1
The H NMR spectrum of 4 is shown in figure 2. The protons of the coordinated pyridine
are strongly shifted to higher field due to the diamagnetic anisotropy of the porphyrin, as
reported for other pyridyl Ru-porphyrin dyads [2]. The ortho-protons of the pyridine moiety
are most affected by the porphyrin ring and resonate as a doublet at 1.88 ppm, while the
meta- and para-protons of the coordinated pyridine appear as triplets at 4.36 ppm and
4.96 ppm, respectively. Restricted rotation of the 4-tert-butylphenyl groups of the porphyrin
is observed for both metalloporphyrin 3 and 4. As a result of the unsymmetrical faces of
the porphyrin, the two sets of ortho and meta aryl protons are chemical shift inequivalent
and show as independent doublets rather than an AA′BB′ system that would result from free
rotation about the aryl–porphyrin bond.
The 13C NMR spectrum of 4 reveals 14 of the expected 15 signals. The three signals for
the coordinated pyridine ring are at 144.6 ppm (ortho), 135.4 (para) and 121.6 ppm (meta).
The carbon of the coordinated carbonyl group resonates at 181.3 ppm, and the 13C NMR
spectrum shows unique resonance for five of the six unique carbons of the 4-tert-butylphenyl
groups. Finally, the C=O vibrational band at 1946 cm−1 dominates the IR spectrum of 4,
very similar to that found in other analogous pyridyl Ru-porphyrin complexes [2b].
The UV–vis spectrum of 4 is depicted in figure 3. The spectrum shows the typical three
bands for a metalloporphyrin, namely the intense Soret-Band of the porphyrin at 415 nm,
Ar
CO
N
N
Ru
N
Ar
N
pyridine,CH2Cl2
rt, 3 h
N
N
N
Ru
CO
Ar
N
H2O
N
Ar
4 (95%)
Ar =
3
Scheme 1. Synthesis of pyridine-porphyrin complex 4.

3092
A.R. Waterloo et al.
Figure 2. 1H NMR spectrum (300 MHz, C6D6) of 4. The resonances assigned to the pyridyl protons are
highlighted.
Figure 3. UV–vis spectrum of 4 in CH2Cl2.
and the two Q-bands at 534 nm and 569 nm. These signals are virtually unchanged from
those of other reported Ru(II)-tetra(aryl)porphyrins with axial ligands, which show analo-
gous absorptions at ca. 415, 532, and 565 nm, regardless of substitution pattern about the
porphyrin [2, 18].

Pentacene–porphyrin dyads
3.3. Synthesis of pentacene–porphyrin dyads
3093
The combination of porphyrin 3 with the pentacene derivatives 1 and 2 was then explored.
Thus, 1 was dissolved in benzene along with a stoichiometric amount of Ru(II)-porphyrin 3
(scheme 2) and the solution stirred at room temperature for an hour. The solution was then
heated to 50 °C and allowed to react for an additional 2 h. After cooling and removal of the
solvent, the resulting purple residue was dissolved in minimal dry, deoxygenated methylene
chloride and precipitated by the addition of a large amount of dry, deoxygenated hexanes.
This gave the pentacene–porphyrin dyad 5 in a yield of 67% as a purple solid. Beginning
with pentacene derivative 2, the analogous procedure gave dyad 6 in 74% yield, also as a
purple solid.
Both 5 and 6 were stable for weeks when stored as solids under ambient laboratory
conditions. In solution, however, the pentacene–porphyrin dyads 5 and 6 were relatively
unstable. The color of the solution changed from purple to red over a period of hours, even
when protected from air and light, in accord with loss of the acene chromophore. This
behavior is consistent with what we, and others [19], have observed in other pentacene sys-
tems, and the constitution of the decomposition product(s) was not pursued. This instability
is disappointing, but not completely unexpected, as Stillmann and coworkers have demon-
strated that the acene chromophore is destroyed rather rapidly in solution [17a], when por-
phyrins are appended to the pro cata positions of 6,13-diphenyl pentacene, likely through
addition of oxygen [19].
3.4. Characterization of pentacene–porphyrin dyads
1H NMR spectroscopy provides evidence for formation of the pentacene–porphyrin dyad 5
1
from pentacene 1 and porphyrin 3. The H NMR spectrum of 5 reflects the diamagnetic
anisotropy from the aromatic porphyrin, as found for 4. Specifically, the ortho-pyridyl
protons of 5 are shifted to 2.42 ppm and 1.95 ppm, relative to uncomplexed 1, in which
they are found at 9.29 and 8.81 ppm, respectively (figure 4). Furthermore, the meta-proton
N
N
N
Ar
N
N
Ar
N
Ru
Ar
N
iPr3Si
N
Ar
CO
Ar
benzene
5 67%
3
+
1 or 2
rt to 50 °C
3 h
or
N
Ar
N
N
N
N
N
Ar =
N
Ru
CO
Ar
N
iPr3Si
Ar
6 74%
Scheme 2. Synthesis of pentacene–porphyrin dyads 5 and 6.

3094
A.R. Waterloo et al.
Figure 4. 1H NMR spectra comparing chemical shifts of pentacene 1 (400 MHz, CDCl3) and pentacene–
porphyrin dyad 5 (300 MHz, C6D6). The resonances assigned to the pyridyl and triazole protons are highlighted
(see http://dx.doi.org/10.1080/00958972.2015.1057711 for color version).
(7.63 ppm) and the para-proton (8.43 ppm) in 1 are shifted upfield upon coordination, and
in dyad 5, they are found at 4.30 ppm (meta-proton) and 5.62 ppm (para-proton). Even the
triazole proton, which is found somewhat downfield at 8.46 ppm in pentacene 1 (but still in
the typical range [16, 20], experiences the diamagnetic anisotropy from the porphyrin ring,
and this proton is shifted upfield to 6.80 ppm in dyad 5.
The spectral characterization of dyad 6 reflects similar trends as established for 5. In the
1H NMR spectrum of 2, the pyridyl protons are doublets at 8.84 and 7.91 ppm, and the
triazole proton resonates as a singlet at 8.48 ppm (figure 5). Upon coordination to Ru(II)-
porphyrin 3, the doublets from the pyridyl group of 6 shift dramatically upfield to 1.86 and
4.72 ppm, while the proton of the triazole ring shifts to 6.30 ppm (figure 5).
IR spectroscopic analyses of 5 and 6 are also consistent with the formation of the pro-
posed supramolecular dyads. Although the shift in each complex is not always significant,

Pentacene–porphyrin dyads
3095
Figure 5. 1H NMR spectra (300 MHz) comparing chemical shifts of pentacene 2 (CDCl3) and dyad 6 (C6D6).
The resonances assigned to the pyridyl and triazole protons are highlighted (+ = silicon grease and * = hexanes)
(see http://dx.doi.org/10.1080/00958972.2015.1057711 for color version).
the vibrational band of the CO group moves from 1943 cm−1 for porphyrin 3 to 1949 cm−1
for 5 and to 1966 cm−1 for 6, as shown in figure 6.
To some extent, solution-state UV–vis analysis sheds light on the electronic properties of
the pentacene–porphyrin assemblies, and this has been investigated for the more stable of
the two dyads, 5 (figure 7). In the UV region, pentacene 1 shows a strong absorption at
308 nm (ε = 240,000) along with a weak absorption at 348 nm. In the low energy region,
absorption bands at 534, 574, and 621 nm are present. Pentacene–porphyrin dyad 5 shows
a strong absorption centered at 309 nm (ε = 210,000) and a weak absorption at 346 nm,
along with the strong Soret-band at 415 nm (ε = 200,000). The low energy region shows
three absorption maxima at 534, 571, and 622 nm. Ultimately, the UV–vis spectrum of 5
demonstrates that there is little change to the electronic absorption properties of either the
individual pentacene or porphyrin moieties, in comparison with dyad 5. Thus, the

3096
A.R. Waterloo et al.
Figure 6. A comparison of the CO stretching vibration of 3 to 5 (top) and 3 to 6 (bottom).
absorption spectrum of 5 can, for the most part, be regarded as a summation of pentacene
and porphyrin absorptions, and the same trend is true for the analysis of dyad 6. It is worth
noting, however, that, in the dyads, the absorptions of the porphyrin chromophore comple-
ment the absorptions of the pentacene groups, leading to a significant increase of absorp-
tions in the mid-UV–vis range versus pentacene alone.

Pentacene–porphyrin dyads
3097
Figure 7. UV–vis spectra of 1 and of 5 as measured in CH2Cl2.
4. Conclusion
Axial coordination of pyridyl-containing pentacenes has been performed. The obtained
pentacene–porphyrin dyads 5 and 6 are soluble in common organic solvents. The dyads 5
and 6 have been characterized through a combination of IR, NMR, UV–vis spectroscopy,
and MS spectrometry and compared to the pyridyl-porphyrin model compound 3. The
UV–vis spectroscopic analysis reveals that the electronic properties of pentacenes are rela-
tively unaffected by axial coordination to the porphyrin. Nevertheless, the UV–vis spectrum
of, e.g. 5, shows that the dyads cover a broad absorption range in the visible part of the
UV–vis spectrum. This could be interesting for optoelectronic devices, although stability
issues of pentacene–porphyrin dyads have to be considered, since solution-state stability for
5 and 6 is somewhat limited. Thus, the practical use of pentacene–porphyrin complexes in
devices will likely require developing a structural motif that provides stabilization to the
acene framework.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Energie Campus Nürnberg (EnCN), “Solar Technologies go Hybrid” (an initiative
of the Bavarian State Ministry for Science, Research and Art), and the Deutsche Forschungsgemeinschaft (DFG)
through the Cluster of Excellence “Engineering of Advanced Materials”.

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A.R. Waterloo et al.
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